CVE Intelligence
High Insecure deserialization

CVE-2026-55009 — Microsoft Exchange Server Elevation of Privilege Vulnerability

Deserialization of untrusted data in Microsoft Exchange Server allows an authorized attacker to elevate privileges locally.

Published Jul 14, 2026 · Updated Oct 15, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: microsoft

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.
How the score was calculated
  • Technical severity (high)+6

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation
Not confirmed
Not in CISA KEV — an absence of record, not proof it is unexploited.
EPSS FIRST
—
No EPSS score.
Affects
Microsoft
Exchange Server and 5 more
CVSS base
7.8
High · 2 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation
None
none · proof-of-concept · active
Automatable
No
can an attacker script the kill-chain steps
Technical impact
Total
partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

  1. Identify exposed assets running the affected vendor/product/version combinations (Microsoft Exchange Server, Microsoft Exchange Server Subscription Edition, Microsoft Microsoft Exchange Server 2016 Cumulative Update 23…).
  2. No fix documented in the sources: reduce exposure and monitor the vendor advisory.
  3. Search available logs for exploit probes matching the vulnerability class (insecure deserialization), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Microsoft Exchange Server version 2016-cumulative_update_23version 2019-cumulative_update_14version 2019-cumulative_update_15 Vulnerable NIST National Vulnerability Database
Microsoft Exchange Server Subscription Edition < 15.02.2562.045 Vulnerable NIST National Vulnerability Database
Microsoft Microsoft Exchange Server 2016 Cumulative Update 23 ≥ 15.01.0.0 and < 15.01.2507.071 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2019 Cumulative Update 14 ≥ 15.02.0.0 and < 15.02.1544.043 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server 2019 Cumulative Update 15 ≥ 15.02.0.0 and < 15.02.1748.048 Vulnerable CVE Program (cve.org) e CISA ADP
Microsoft Microsoft Exchange Server Subscription Edition RTM ≥ 15.02.0.0 and < 15.02.2562.045 Vulnerable CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Local
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
7.8 CVSS 3.1 High 1.8 5.9 secure@microsoft.com via NIST National Vulnerability Database
7.8 CVSS 3.1 High — — microsoft via CVE Program (cve.org) e CISA ADP
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (score shown at the top: secure@microsoft.com)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-502 · Deserialization of Untrusted Data

Other CVEs with this weakness →

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

  • Local/physical vector: focus on endpoint telemetry (EDR) from affected hosts.
  • Monitor serialized objects in requests and unexpected processes spawned by the runtime.
endpoint telemetry (EDR)application logs

Timeline

What happened to this CVE, newest first.

  • 2026
  • Affected products added · microsoft/microsoft exchange server 2016 cumulative update 23, microsoft/microsoft exchange server 2019 cumulative updat
    Oct 15, 2026 · CVE Program (cve.org) e CISA ADP
  • SSVC assessment updated
    Oct 15, 2026 · CVE Program (cve.org) e CISA ADP
  • Record published
    Oct 15, 2026
  • CVSS changed · 7.8
    Oct 15, 2026 · NIST National Vulnerability Database
  • Source status changed · → Analyzed
    Oct 15, 2026 · NIST National Vulnerability Database
  • Record created in this database
    Oct 15, 2026 · NIST National Vulnerability Database
  • CVE published
    Jul 14, 2026 · microsoft

Source activity

Repeated readings are counted rather than listed.

  • 1× Official description updated · Oct 15, 2026 – Oct 15, 2026

References

1 on the record

Elsewhere on this site

Microsoft
every CVE for this vendor
Insecure deserialization
same class
CWE-502
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-87696 Microsoft Exchange Server Elevation of Privilege Vulnerability 2d
  • CVE-2026-87695 Microsoft Exchange Server Elevation of Privilege Vulnerability 2d
  • CVE-2026-87692 Microsoft Exchange Server Elevation of Privilege Vulnerability 2d
  • CVE-2026-77910 Microsoft Exchange Server Elevation of Privilege Vulnerability 2d
  • CVE-2026-70571 Microsoft Exchange Server Remote Code Execution Vulnerability 2d
  • CVE-2026-70279 Microsoft Exchange Server Spoofing Vulnerability 2d

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No FIRST EPSS score.
  • No fixed version, patch or mitigation documented.
  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-55009 is "Microsoft Exchange Server Elevation of Privilege Vulnerability", rated High. It affects Exchange Server (Microsoft); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 7.8 describes technical severity. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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